Reversible protein tyrosine phosphorylation plays an essential role in regulating cell proliferation. Many receptor-linked protein tyrosine kinases (PTKs), including the epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) receptors, induce cell proliferation following ligand binding. The dysregulation of these PTKs can result in uncontrolled cell proliferation characteristic of neoplastic transformation. Indeed, about a third of all known oncogenes are PTKs. Regulation of protein tyrosine phosphorylation, however, requires the concerted activities of both PTKs and protein tyrosine phosphatases (PTPase). Although PTKs have been well characterized, it is only recently that a family of protein tyrosine phosphatases (PTPases) has been identified. One of these PTPases, designated LAR, possesses an extracellular domain whose structure resembles a cell adhesion receptor. Intriguingly, this putative adhesion receptor domain can be shed from the cell surface under conditions of high density cell culture. Taken together, these results suggest that the PTPase activity of LAR might be regulated by cell-cell interactions, and that LAR may play a role in cell-cell contact inhibition. Furthermore, the possibility that LAR is a tumor suppressor gene is supported by the finding that LAR is expressed in the same cell types that give rise to lung, breast, skin and colon cancers. The overall aim of this proposal is to increase our understanding of the role protein tyrosine phosphatases play in regulating cell proliferation. The Specific Aims are: i) To determine the functional significance of LAR structural maturation, ii) To identify the physiological substrate(s) dephosphorylated by the LAR PTPase, iii) To identify the extracellular ligand of LAR, and iv) To determine the role of LAR gene transcription in regulating LAR function. The health-relatedness of the project is to increase our understanding of how cells are signalled to proliferate or stop proliferating. These aims will be accomplished by: i) defining by mutational analysis the amino acid sequences required for LAR processing and shedding; ii) determining what signals induce LAR shedding, and whether these signals alter PTPase activity; iii) establishing LAR-negative cell lines to determine the functional role of the LAR PTPase; iv) identifying LAR substrates using immunochemical techniques; v) using purified, soluble LAR as an affinity probes to isolate the LAR ligand; and vi) determining the structure and regulation of the LAR gene promoter.